The tubulysins are members of a new class of natural products isolated from myxobacterial species (F. Sasse, et al., J. Antibiot. 2000, 53, 879-885). As cytoskeleton interacting agents, the tubulysins are mitotic poisons that inhibit tubulin polymerization and lead to cell cycle arrest and apoptosis (H. Steinmetz, et al., Chem. Int. Ed. 2004, 43, 4888-4892; M. Khalil, et al., ChemBioChem. 2006, 7, 678-683; G. Kaur, et al., Biochem. J. 2006, 396, 235-242). Tubulysins are extremely potent cytotoxic molecules, and exceed the cell growth inhibition of many other clinically relevant traditional chemotherapeutics, including epothilones, paclitaxel, and vinblastine. Furthermore, they are potent against multidrug resistant cell lines (A. Domling, et al., Mol. Diversity 2005, 9, 141-147). These compounds show high cytotoxicity tested against a panel of cancer cell lines with IC50 values in the low picomolar range; thus, they are of interest as potential anticancer therapeutics. However, tubulysins have been reported to exhibit a narrow, or in some cases nonexistent, therapeutic window such that disease treatment with tubulysins is hampered by toxicity and other unwanted side effects. Accordingly, tubulysins have been conjugated with targeting agents to improve their therapeutic window. A total synthesis of tubulysin D possessing C-terminal tubuphenylalanine (RA═H) (H. Peltier, et al., J. Am. Chem. Soc. 2006, 128, 16018-16019) has been reported. Recently, a modified synthetic protocol toward the synthesis of tubulysin B (RA═OH) (O. Pando, et al., Org. Lett. 2009, 11, 5567-5569) has been reported. However, attempts to follow the published procedures to provide larger quantities of tubulysins were unsuccessful, being hampered in part by low yields, difficult to remove impurities, the need for expensive chromatographic steps, and/or the lack of reproducibility of several steps. The interest in using tubulysins for anticancer therapeutics accents the need for reliable and efficient processes for preparing tubulysins, and analogs and derivatives thereof. Therefore, there is a need for tubulysin derivatives, tubulysin analogs, and other tubulysin conjugate intermediates that are useful for preparing such targeted conjugates.
Tubulysin derivatives useful for preparing vitamin receptor binding tubulysin conjugates (also referred to herein as tubulysin linker derivatives) are described herein. Structurally, tubulysin linker derivatives include linear tetrapeptoid backbones, including illustrative compounds having the following formula
and salts thereof, wherein
Ar1 is optionally substituted aryl or optionally substituted heteroaryl;
Ar2 is optionally substituted aryl or optionally substituted heteroaryl;
L is selected from the group consisting of
where p is an integer from about 1 to about 3, m is an integer from about 1 to about 4, and * indicates the points of attachment;
Ra, Rb, and R are each independently selected in each instance from the group consisting of hydrogen and alkyl; or any two of Ra, Rb, and R are taken together with the attached carbon atom(s) to form a carbocyclic ring; RAr represents hydrogen, or 1 to 4 substituents each independently selected from the group consisting of amino or derivatives thereof, hydroxy or derivatives thereof, halo, thio or derivatives thereof, nitro, sulfonic acids and derivatives thereof, carboxylic acids and derivatives thereof, and alkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, and heteroarylheteroalkyl, each of which is optionally substituted;
X is hydrogen; or X is alkyl or alkenyl, each of which is optionally substituted; or X is R16C(O)CH(R17)CH2; where R17 is C(O)R16, C(O)OR16, or CN; where R16 is independently in each instance alkyl, alkenyl, cycloalkyl, cycloalkenyl, heteroaryl, arylalkyl, or heteroarylalkyl, each of which is optionally substituted; or X is CH2QR18; where Q is N, O, or S; and R18 is alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, arylalkyl, heteroaryl, or heteroarylalkyl, each of which is optionally substituted, or R18 is acyl, sulfonyl, or phosphonic acid or a derivative thereof;
R4 is optionally substituted alkyl or optionally substituted cycloalkyl;
R3 is optionally substituted alkyl;
R5 and R6 are each independently selected from the group consisting of optionally substituted alkyl and optionally substituted cycloalkyl;
R7 is optionally substituted alkyl;
RAr represents hydrogen, or 1 to 4 substituents each independently selected from the group consisting of amino or derivatives thereof, hydroxy or derivatives thereof, halo, thio or derivatives thereof, nitro, sulfonic acids and derivatives thereof, carboxylic acids and derivatives thereof, and alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, and heteroarylheteroalkyl; and n is 1, 2, 3, or 4.
In another embodiment, X is Y—CH2, where Y is R2C(O)O or R12O; R2 is selected from the group consisting of optionally substituted alkyl and optionally substituted cycloalkyl; and R12 is alkyl, alkenyl, alkynyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl or heteroarylalkyl, each of which is optionally substituted.
In another embodiment, compounds of the formula
and salts thereof, are described. In another embodiment, the tubulysin linker derivative has formula T1
or a salt thereof. In another embodiment, the tubulysin linker derivative has formula T2
or a salt thereof.
In another embodiment, in any of the embodiments described herein Ar2 is optionally substituted aryl.
In another embodiment, in any of the embodiments described herein Ar2 is optionally substituted heteroaryl.
Another illustrative group of tubulysins described herein are more particularly comprised of one or more non-naturally occurring or hydrophobic amino acid segments, such as N-methyl pipecolic acid (Mep), isoleucine (Ile),
and analogs and derivative of each of the foregoing. Derivatives and analogs of tubuvaline include compounds of the following formula,
wherein R4, R5 and R6 are as described in any of the embodiments described herein. Derivatives and analogs of tubutyrosine or tubuphenylalanine include compounds having formula,
wherein R3 and Ar1 are as described in any of the embodiment described herein. A common feature in the molecular architecture of potent natural occurring tubulysins is the acid and/or base sensitive N-acyloxymethyl substituent (or a N,O-acetal of formaldehyde) represented by R2CO2CH2 in the formula (T1).
In another embodiment, the compounds described herein are NHNH—C(O)O-L-SS—Ar2 derivatives of naturally occurring tubulysins. An illustrative group of tubulysin derivatives described herein are those having formula 1.
 Formula 1, Structures of several tubulysin derivativesTubulysinRAR2AOHCH2CH(CH3)2BOHCH2CH2CH3COHCH2CH3DHCH2CH(CH3)2EHCH2CH2CH3FHCH2CH3GOHCH═C(CH3)2HHCH3IOHCH3
Processes for preparing tubulysins, and analogs and derivatives thereof, are also described in WO 2012/019123, the disclosure of which is incorporated herein by reference in its entirety.
The formation of tubulysins conjugated to vitamin receptor binding moieties for targeted and/or selective delivery to cell populations expressing, overexpressing or selectively expressing cell surface vitamin receptors necessitates further modification of the highly toxic tubulysins. Described herein are improved processes for making natural tubulysins analogs or derivatives, which are useful for preparing vitamin receptor binding tubulysin conjugates including compounds of formula (T) and formula (I). Vitamin receptor binding conjugates of tubulysins are described in U.S. Patent Publication 2010/0048490, the disclosure of which is incorporated herein by reference in its entirety.
In one illustrative embodiment of the invention, processes for derivatives or analogs of natural tubulysins including compounds of formula (T). In another embodiment, vitamin receptor binding conjugates of tubulysins are described. The processes include one or more steps described herein. In another embodiment, a process is described for preparing a compound of formula B, wherein R5 and R6 are as described in the various embodiments herein, such as each being independently selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R8 is C1-C6 n-alkyl; wherein the process comprises the step of treating a compound of formula A with a silylating agent, such as triethylsilyl chloride, and a base, such as imidazole in an aprotic solvent.
It is to be understood that R5 and R6 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula C, wherein R5 and R6 are as described in the various embodiments herein, such as each being independently selected from optionally substituted alkyl or optionally substituted cycloalkyl; R8 is C1-C6 n-alkyl; and R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; wherein the process comprises the step of treating a compound of formula B with a base and a compound of the formula ClCH2OC(O)R2 in an aprotic solvent at a temperature below ambient temperature, such as in the range from about −78° C. to about 0° C.; wherein the molar ratio of the compound of the formula ClCH2C(O)R2 to the compound of formula B is from about 1 to about 1.5.
It is to be understood that R2, R5 and R6 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula D, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R8 is C1-C6 n-alkyl; R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl; wherein the process comprises the steps of
a) preparing a compound of formula (E1) where X1 is a leaving group from a compound of formula E; and
b) treating a compound of formula C under reducing conditions in the presence of the compound of formula E1.
It is to be understood that R2, R5, R6, and R7 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula F, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl; wherein the process comprises the step of treating compound D with a hydrolase enzyme.

In another embodiment, a process is described for preparing a compound of formula F, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl; wherein the process comprises the step of treating compound D with a trialkyltin hydroxide (e.g. trimethyltin hydroxide). It is to be understood that R2, R5, R6, and R7 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula AF, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl; wherein the process comprises the step of contacting compound D with an alcohol, R12OH, where R12 is alkyl, alkenyl, alkynyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl or heteroarylalkyl, each of which is optionally substituted; and a transesterification catalyst. In one embodiment the transesterification catalyst is trifluoroacetic acid (TFA). In another embodiment, the transesterification catalyst is selected from the group consisting of (R13)8Sn4O2(NCS)4, (R13)2Sn(OAc)2, (R13)2SnO, (R13)2SnCl2, (R13)2SnS, (R13)3SnOH, and (R13)3SnOSn(R13)3, where R13 is independently selected from alkyl, arylalkyl, aryl, or cycloalkyl, each of which is optionally substituted. In another embodiment, the transesterification catalyst is (R13)2SnO. Illustrative examples of R13 include methyl, n-butyl. n-octyl, phenyl, o-MeO-phenyl, p-MeO phenyl, phenethyl, and benzyl.
It is to be understood that R5, R6, R12, and R7 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a transesterification product of each of the compounds of formula A and/or B, wherein R5, R6, and R8 are as described in the various embodiments herein, and where R12 is different from R9; wherein the process comprises the step of contacting compound B with an alcohol, R12OH, where R12 is alkyl, alkenyl, alkynyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl or heteroarylalkyl, each of which is optionally substituted; and a transesterification catalyst. Illustratively, the transesterification catalyst is selected from the group consisting of (R13)8Sn4O2(NCS)4, (R13)2Sn(OAc)2, (R13)2SnO, (R13)2SnCl2, (R13)2SnS, (R13)3SnOH, and (R13)3SnOSn(R13)3, where R13 is independently selected from alkyl, arylalkyl, aryl, or cycloalkyl, each of which is optionally substituted. In another embodiment, the transesterification catalyst is (R13)2SnO. Illustrative examples of R13 include methyl, n-butyl. n-octyl, phenyl, o-MeO-phenyl, p-MeO phenyl, phenethyl, and benzyl.
It is to be understood that R5, R6, and R12 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula G, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl; wherein the process comprises the step of treating the silyl ether of compound F with a non-basic fluoride containing reagent.
It is to be understood that R2, R5, R6, and R7 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula AG, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl; wherein the process comprises the step of contacting compound F with an alcohol, R12OH, where R12 is alkyl, alkenyl, alkynyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl or heteroarylalkyl, each of which is optionally substituted; and a transesterification catalyst. In one embodiment the transesterification catalyst is trifluoroacetic acid (TFA). In another embodiment, the transesterification catalyst is selected from the group consisting of (R13)8Sn4O2(NCS)4, (R13)2Sn(OAc)2, (R13)2SnO, (R13)2SnCl2, (R13)2SnS, (R13)3SnOH, and (R13)3SnOSn(R13)3, where R13 is independently selected from alkyl, arylalkyl, aryl, or cycloalkyl, each of which is optionally substituted. In another embodiment, the transesterification catalyst is (R13)2SnO. Illustrative examples of R13 are methyl, n-butyl. n-octyl, phenyl, o-MeO-phenyl, p-MeO phenyl, phenethyl, and benzyl.
It is to be understood that R2, R5, R6, R7, and R12 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula BG, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 is as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R12 is as described in the various embodiments herein, such as being selected from alkyl, alkenyl, alkynyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl or heteroarylalkyl, each of which is optionally substituted; and R7 is optionally substituted alkyl; wherein the process comprises the step of contacting compound AF with a metal hydroxide or carbonate. Illustrative examples of a metal hydroxide or carbonate include LiOH, Li2CO3, NaOH, Na2CO3, KOH, K2CO3, Ca(OH)2, CaCO3, Mg(OH)2, MgCO3, and the like.
It is to be understood that R5, R6, R7, and R12 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula H, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 and R4 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl; wherein the process comprises the step of treating a compound of formula G with an acylating agent of formula R4C(O)X2, where X2 is a leaving group.
It is to be understood that R2, R4, R5, R6, and R7 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a compound of formula AH, wherein R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R2 and R4 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R12 is as described in the various embodiments herein, such as being selected from alkyl, alkenyl, alkynyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl or heteroarylalkyl, each of which is optionally substituted; and R7 is optionally substituted alkyl; wherein the process comprises the step of treating a compound of formula BG with an acylating agent of formula R4C(O)X2, where X2 is a leaving group.
It is to be understood that R4, R5, R6, and R7 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a tubulysin linker derivative of formula (T1), wherein Ar1 is optionally substituted aryl; Ar2 is optionally substituted aryl or optionally substituted heteroaryl; L is selected from the group consisting of
where
p is an integer from about 1 to about 3, m is an integer from about 1 to about 4, and * indicates the points of attachment;
Ra, Rb, and R are each independently selected in each instance from the group consisting of hydrogen and alkyl; or any two of Ra, Rb, and R are taken together with the attached carbon atom(s) to form a carbocyclic ring;
RAr represents 0 to 4 substituents selected from the group consisting of amino, or derivatives thereof, hydroxy or derivatives thereof, halo, thio or derivatives thereof, alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, heteroarylheteroalkyl, nitro, sulfonic acids and derivatives thereof, carboxylic acids and derivatives thereof; R1 is hydrogen, optionally substituted alkyl, optionally substituted arylalkyl or a pro-drug forming group; R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R3 is optionally substituted alkyl; R2 and R4 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl;
wherein the process comprises the step of forming an active ester intermediate from a compound of formula H; and reacting the active ester intermediate with a compound of the formula I to give a compound of the formula T.
It is to be understood that Ar1, Ar2, R1, R2, R4, R5, R6, and R7 may each include conventional protection groups on the optional substituents.
In another embodiment, a process is described for preparing a tubulysin linker derivative of formula (T2), wherein Ar1 is optionally substituted aryl; Ar2 is optionally substituted aryl or optionally substituted heteroaryl; L is selected from the group consisting of
where
p is an integer from about 1 to about 3, m is an integer from about 1 to about 4, and * indicates the points of attachment;
Ra, Rb, and R are each independently selected in each instance from the group consisting of hydrogen and alkyl; or any two of Ra, Rb, and R are taken together with the attached carbon atom(s) to form a carbocyclic ring;
RAr represents 0 to 4 substituents selected from the group consisting of amino, or derivatives thereof, hydroxy or derivatives thereof, halo, thio or derivatives thereof, alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, heteroarylheteroalkyl, nitro, sulfonic acids and derivatives thereof, carboxylic acids and derivatives thereof;
R1 is hydrogen, optionally substituted alkyl, optionally substituted arylalkyl or a pro-drug forming group; R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R3 is optionally substituted alkyl; R2 and R4 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl;
wherein the process comprises the step of contacting compound T, with an alcohol, R12OH, where R12 is alkyl, alkenyl, alkynyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl or heteroarylalkyl, each of which is optionally substituted; and a transesterification catalyst.
In one embodiment the transesterification catalyst is trifluoroacetic acid (TFA). In another embodiment, the transesterification catalyst is selected from the group consisting of (R13)8Sn4O2(NCS)4, (R13)2Sn(OAc)2, (R13)2SnO, (R13)2SnCl2, (R13)2SnS, (R13)3SnOH, and (R13)3SnOSn(R13)3, where R13 is independently selected from alkyl, arylalkyl, aryl, or cycloalkyl, each of which is optionally substituted. In another embodiment, the transesterification catalyst is (R13)2SnO. Illustrative examples of R13 are methyl, n-butyl. n-octyl, phenyl, o-MeO-phenyl, p-MeO phenyl, phenethyl, and benzyl. It is to be understood that Ar1, Ar2, R1, R2, R4, R5, R6, R7, and R12 may each include conventional protection groups on the optional substituents.

In another embodiment, a process is described for preparing a tubulysin linker derivative of formula (T2), wherein Ar1 is optionally substituted aryl; Ar2 is optionally substituted aryl or optionally substituted heteroaryl; L is selected from the group consisting of
where
p is an integer from about 1 to about 3, m is an integer from about 1 to about 4, and * indicates the points of attachment;
Ra, Rb, and R are each independently selected in each instance from the group consisting of hydrogen and alkyl; or any two of Ra, Rb, and R are taken together with the attached carbon atom(s) to form a carbocyclic ring;
RAr represents 0 to 4 substituents selected from the group consisting of amino, or derivatives thereof, hydroxy or derivatives thereof, halo, thio or derivatives thereof, alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, heteroarylheteroalkyl, nitro, sulfonic acids and derivatives thereof, carboxylic acids and derivatives thereof;
R1 is hydrogen, optionally substituted alkyl, optionally substituted arylalkyl or a pro-drug forming group; R5 and R6 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; R3 is optionally substituted alkyl; R2 and R4 are as described in the various embodiments herein, such as being selected from optionally substituted alkyl or optionally substituted cycloalkyl; and R7 is optionally substituted alkyl;
wherein the process comprises the step of forming an active ester intermediate from a compound of formula AH; and reacting the active ester intermediate with a compound of the formula I to give a compound of the formula T2.
It is to be understood that Ar1, Ar2, R1, R12, R3, R4, R5, R6, and R7 may each include conventional protection groups on the optional substituents.
It is to be understood that the acyloxymethyl group (R2—C(O)—CH2) present on any of compounds C, D, F, G, H, and T1 may be converted into the corresponding ether group (R12—O—CH2), or other group (X or Y—CH2) using the process of contacting the compound with trifluoroacetic acid (TFA), as described herein, and also as described in WO 2009/055562, the disclosure of which is incorporated herein by reference. Accordingly, the following compounds are also described herein

It is to be further understood that each of XC, XD, XF, XG, and XH can be used in the processes described herein in place of each of C, D, F, G, and H, respectively, to prepare the corresponding compound having an ether group (R12—O—CH2), or other group (X).
It is to be further understood that each of YC, YD, YF, YG, and YH can be used in the processes described herein in place of each of C, D, F, G, and H, respectively, to prepare the corresponding compound having an ether group (R12—O—CH2), or other group (Y—CH2).